3 Phase with only 2 transformers?

LLSolutions

Senior Member
This discussion came up at work. We all agreed on how a typical 3p 120/240v delta with a high leg and center tap in one of the windings for the neutral works. However someone said they use to do a 3 phase system that also had a high leg but only used 2 transformers. The discussion was all based on pole mounted utility transformers. I couldn't think of how that would work, unless the phases are not truly 120 degrees apart. So is one winding center tapped and that gives you 2 "phases" and the the other would be 180 degrees off the center of the the one thats tapped making it 90 degrees off each of the others? I would LOOOOVE a drawing of this. Thanks guys!!!
 

Smart $

Esteemed Member
This discussion came up at work. We all agreed on how a typical 3p 120/240v delta with a high leg and center tap in one of the windings for the neutral works. However someone said they use to do a 3 phase system that also had a high leg but only used 2 transformers. The discussion was all based on pole mounted utility transformers. I couldn't think of how that would work, unless the phases are not truly 120 degrees apart. So is one winding center tapped and that gives you 2 "phases" and the the other would be 180 degrees off the center of the the one thats tapped making it 90 degrees off each of the others? I would LOOOOVE a drawing of this. Thanks guys!!!
It's called "open delta". If you Google this term, you will get all kinds of info.

Picture three transformers providing 240/120V 3? 4W service that you mention. Now remove one not center-tapped. The voltage and phase across the terminals where the removed transformer was formerly connected is still the same.

BTW, POCO's still do this.
 

ATSman

Senior Member
Open Delta Configuration

Open Delta Configuration

This discussion came up at work. We all agreed on how a typical 3p 120/240v delta with a high leg and center tap in one of the windings for the neutral works. However someone said they use to do a 3 phase system that also had a high leg but only used 2 transformers. The discussion was all based on pole mounted utility transformers. I couldn't think of how that would work, unless the phases are not truly 120 degrees apart. So is one winding center tapped and that gives you 2 "phases" and the the other would be 180 degrees off the center of the the one thats tapped making it 90 degrees off each of the others? I would LOOOOVE a drawing of this. Thanks guys!!!
Control circuits have used open delta connections since I don't know how long. It also is used in powering up a load, although the KVA rating would be reduced compared to using 3 single phase transformers. Rather than me rambling on, copy & paste these links:

http://www.federalpacific.com/university/transbasics/chapter3.html

http://www.google.com/search?q=open+delta+configuration&hl=en&rlz=1T4ADFA_enUS421US421&prmd=imvnsb&tbm=isch&tbo=u&source=univ&sa=X&ei=DCxpT5TkMqG0iQKCkbj9Bg&ved=0CEUQsAQ&biw=806&bih=432
 

broadgage

Senior Member
To add to the points above, for small 3 phase services, utilities often use open delta as only 2 transformers need be purchased and not 3.
Later, a third transformer may added when the load justifies this. Initialy buying two transformers each of say 20KVA, and then adding another when needed, is clearly cheaper than 3 smaller transformers, all 3 of which will need replacement with larger ones when the load grows.

The capacity of two transformers in open delta is 57% of the capacity of 3 similar transformers, and not 66% as one might expect.

Although 208 volts is available from an open delta transformer bank, the connection of any significant 208 volt load is unwise as extra losses are caused.

Single phase 240 volt loads should be spread eqaully between the two transformers, and NOT connected accros the "missing" phase.
A single phase 240 volt load would work between A and B , but the voltage drop and transformer losses would be increased.
 

Open Neutral

Senior Member
To add to the points above, for small 3 phase services, utilities often use open delta as only 2 transformers need be purchased and not 3.
Later, a third transformer may added when the load justifies this. Initialy buying two transformers each of say 20KVA, and then adding another when needed, is clearly cheaper than 3 smaller transformers, all 3 of which will need replacement with larger ones when the load grows.

In my project, PG&E supplies a "duplex" pad xfmr. The center-tapped "lighter" is 25KVA, the "stinger" is only 10. Overhead, you can see 2 different sized pole pigs all around the area.

Amazingly, even when the power drop is solely for a 3∅ well; they use a bigger lighter. [The area has many wells remote from the associated houses; with deeded easements for wells and water lines. In other cases tankage is on an easement to get gravity feed to the house.]
 

LLSolutions

Senior Member
Makes sense, when I was picturing it my head I was still imagining only using 2 of the phases on the incoming side of the transformer. Thanks guys!!!
 
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